1. Field of the Invention
This invention relates to an improved foam drilling fluid composition and method of use thereof in drilling high temperature subterranean reservoirs. More particularly, this invention relates to such a composition and method for drilling such reservoirs with reduced damage to the drilling apparatus and to the reservoir itself.
2. Description of the Prior Art
Drilling wells into subterranean reservoirs containing a high temperature fluid presents many problems concerning the choice of a drilling fluid. Conventional aqueous base drilling fluid containing water, clay and various additives is generally used in drilling the first 2 to 4,000 feet of hole through the usually somewhat incompetent strata above the strata containing the high temperature fluid. Casing is cemented in place over this portion of the hole and drilling is continued using any one of a number of drilling fluids. Problems arise at lower depths when the drill bit penetrates those strata of the reservoir containing a high temperature fluid, which strata often lie several thousand feet below the surface. Such strata are sometimes highly porous and permeable. If conventional drilling fluid is used, the weight of the high gravity column of drilling fluid in the well is so great that a considerable amount of the fluid invades the strata. This necessitates use of a large additional volume of fluid and results in partial plugging of the strata by the clay and other solids in the invading fluid. When the well is subsequently completed, this plugging decreases the amount of fluid which can be produced via or injected into the well. If a relatively light-weight drilling fluid, such as air or another gas, is used, a high rate of circulation of the drilling fluid through the system is required to circulate cuttings out of the hole. This high rate of circulation of the drilling fluid together with the fluids produced from the reservoir, result in corrosion and erosion problems. The high velocity gaseous fluids erode the unconsolidated portions of the reservoir to such an extent that hole stability cannot be achieved. In many cases, even the drill pipe is eroded.
One solution to these problems has been to drill strata containing a high temperature fluid with aqueous foam drilling fluid. Use of this light-weight foam drilling fluid comprising a mixture of water, a gas such as air, a foaming agent and an erosion and corrosion inhibitor, results in increased penetration rates, reduced bit wear and reduced reservoir damage due to erosion by the drilling fluid as compared with air. However, these reservoirs have certain characteristics that make them difficult to drill, even with the previously used foam drilling fluids. For example, such reservoirs of heterogeneous permeability often contain strata of such high permeability of low pressure that they are invaded and partially plugged even by a low density foam drilling fluid. It is through these high permeability strata that most of the fluids produced from or injected into the reservoir must pass. Since it is difficult to effect complete removal of the drilling fluid from the strata following the drilling operation, it is known to add a fluid loss control additive to the foam drilling fluid to reduce the amount of drilling fluid penetrating the strata. U.S. patent application Ser. No. 639,652 filed Dec. 12, 1975, now U.S. Pat. No. 4,013,568, by Fischer et al. describes lignite as a fluid loss control additive for a foam drilling fluid.
Since the reservoir being drilled is at a high temperature and the foam drilling fluid is relatively fragile compared to previously used aqueous drilling muds, there is also a need to increase the stability of the foam. This is particularly true under drilling conditions where the formation pressure is greater than the pressure of the column of drilling fluid. Under these conditions, some of the hot formation fluids enter the well and mix with the circulating foam drilling fluid. One approach has been to use a particular foaming agent which forms a foam which is especially stable at high temperatures. U.S. patent application Ser. No. 621,685 filed Oct. 14, 1975, now U.S. Pat. No. 4,036,764 by Fisher et al. discloses sodium lauryl sulfoacetate and similar compounds as such foaming agents. However, need exists for a still more stable foam drilling fluid, e.g., one that can be made using previously used foaming agents.
Accordingly, a principal object of this invention is to provide a lightweight drilling fluid and method of use thereof suitable for drilling a subterranean reservoir containing a high temperature fluid.
Another object of the invention is to provide such a composition having improved stability.
A further object of the invention is to provide such a method and composition wherein the composition has a reduced tendency to invade the reservoir surrounding the well and hence causes reduced reservoir damage.
A still further object of the invention is to provide such a method and composition for drilling geothermal reservoirs containing wet steam having a temperature of 350.degree. F. and above.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.